The present invention relates to an engine control system and, more particularly, to an engine control system suited for backup when an crank angle sensor becomes abnormal.
As a backup method for an ignition control when a crank angle sensor is abnormal, a conventional engine control is accomplished by providing not only a crank angle sensor for normal control but also a backup crank angle sensor so that the output of the backup crank angle sensor is switched to calculate the ignition timing when an abnormality of the normal crank angle sensor is detected, as disclosed in Japanese Patent Laid-Open No. 62-225770. The ignition timing control as mentioned above, however, is not properly accomplished when the normal crank angle sensor is switched to the backup crank angle sensor when in an abnormal operation. As a result, the normal ignition timing signal cannot be outputted at an abrupt interruption when a position signal outputted from the crank angle sensor becomes deficient but is instantly retrieved. Thus, rotational fluctuations are caused to invite an engine stall in the worst case.